The big moment is finally here. Ginger and her drones inside the planet's most magnificent hidden world. We went right up to the brink yesterday. Today ginger will take us inside. She's been trekking for days and we are so excited to get inside with ginger right now. Where exactly are you, ginger? S Oh, George, I am way down here from the perspective you can see me about 4 feet below the top of one of the world's largest caverns inside a mountain inside a jungle in central Vietnam and this, I have to be honest with you, has been one of or the, I would say, most difficult, dangerous and gritiest assignments I have ever been a part of. Let me explain. It may look like a white sand beach where we're standing. Staying in these tents and cooking over fires. This, again, is very remote. We are not close to any civilization but we've got a lot of folks around us, a great team that has come together because we had to find the hidden world that was only a third of a mile from here and there was only found a couple of decades ago. But really was not discovered and explored until 2009. So this is brand-new to our world. The subterranean surreal life that has been revealed and now you get to be a part of it including the awesome images you'll see from our friends at dji drones. A clause to froebic crevice. You have to crawl to get into the biggest cave in the world. Okay. This is small. Then it opens up. I don't want to miss this part. And drops down and down and down. I can't see beyond the narrow beam of my headlamp but I am keenly aware those ropes are here because a fall would be disastrous. Sorry. This place does not look real. At the bottom I feel like I've landed on another planet. A subterranean rushing river. Now I find myself in mother nature's sculpture garden filled with prehistoric bowelers and stalagmites built up from calcium deposited from dripping water. This is a stalagmite. We're inside a stalagmite where the water is worn away all the bottom. Reporter: After a grueling hour an illuminating light, a thousand feet overhead. Spectacular. Every turn there's something else. This colossal sky light where the chamber ceiling collapsed is called a doe line breathing light into this underground world, a lush green oasis, the camp's white beaches lit up below. It looks like avatar in here. Doesn't feel real. Reporter: This is that hidden world we've been chasing. Mountain river cave, the planet's largest and visited by fewer explorers than outer pace. Just look at this. It is unbelievable. I'm standing on a could last Sal stalagmite. The jewel in the crown of this cave. The moment that took my breath away and it's also the moment I realized we're not even halfway there yet. We press on and suddenly I'm in a jungle under a second monster skylight. Not atop of a mountain but inside it. Way inside it. More than 800 feet below the surface. You don't normally get trees and youngles in caves. Reporter: Youngle plants and animals from above adapting to this unique subterranean netherworld. We've come across monkeys sticks thrown at me by monkeys. In here. Throwing sticks. What a view from those camera. This mammoth crater big enough to park two 747s inside and tall enough to fit an 80-story skyscraper. The cave's flowing river, the creator of this alien world carving its way through a limestone mountain for 2 million years forming the largest underground chamber on Earth. More than 1.3 billion cubic feet of subterranean awesomeness. That's more than 15,000 olympic-size swimming pools. Appropriate since five months out of the year this underground marvel is underwater. It goes 80 meters tall and 100 meters wide and flowing very, very fast. No way you can get anywhere near this area. Reporter: Danger is everywhere here. Alongside breathtaking beauty like a fantasy world come to life. The punishing journey worth every ounce of sweat. What gives me goose bumps are the still unexplored passages that branch off our route tunneling deeper. Good morning, America. Welcome. The majesty of this place will take you on such an emotional journey. I feel superior because I climbed this. I'm part of history but I feel inferior because it's something this grandiose was just found imagine what else is out there. ? Beauty in the world ? This adventure is only for the elite. And their emphasis is on conservation and sustainability. As roughs it's been it was totally worth it now that you can see those images. We sure can looking down on you now. I know you have the drone there. Can you send it up the wall and give us a sense of how deep under underyou are right now so we get a sense of the scale? Yeah, at this point we are almost 400 feet under ground but this is one of the more shallow places. There are places within that mountain river cave that are 1,000 feet below or more and they haven't even explored all of that. That's the crazy part. Wow, that is just incredible. The technical crew to be able to pull this off too. Kudos. Well done. We'll have a lot more from

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